Well Treating Method to Prevent or Cure Lost-Circulation

ABSTRACT

A method of preventing or treating lost circulation during the drilling of a well comprises the addition, at concentration ranging between about 0.5 and 6 pounds per barrel, of water-dispersible fibers having a length between about 10 and 25 mm, for instance glass or polymer fibers, to a pumped aqueous base-fluid including solid particles having an equivalent diameter of less than 300 μm. The base-fluid with the solid particles can be the drilling fluid or a small-volume pill specially pumped for curing lost circulation.

FIELD OF THE INVENTION

The present invention relates to the methods of preventing or curinglost circulation problems encountered during the drilling of wells suchas oil, gas or geothermal wells, or the like.

BACKGROUND OF THE INVENTION

Lost circulation is defined as the total or partial loss of drillingfluids or cement into highly permeable zones, cavernous formations, andnatural or induced fractures during drilling or cementing operations.The consequences of lost circulation can include:

-   -   Blowout, due to drop in fluid level in the well (loss of        hydrostatic head);    -   Stuck pipe due to poor cutting removal;    -   Zonal isolation failure due to insufficient cement filling;    -   Increased cost due to loss of drilling fluids or cement,        increased rig time and remedial cementing operations;    -   Formation damage due to losses to the producing zone; and    -   Loss of the well.    -   The severity of lost circulation can vary from minor (<10 bbl        (1.5 m³)/hr) to severe (total loss of fluids, unable to keep        hole full or obtain returns to surface).

One approach to dealing with lost circulation is to add materials (“lostcirculation materials” or “LCMs”) to the fluid which bridge or blockseepage into the formation. LCMs generally fall into four main types:

-   -   Granular (e.g. ground nut shells, plastics, or limestone);    -   Lamellar (e.g. cellophane flakes);    -   Fibrous (e.g. sawdust, hay, glass fibers); and    -   Encapsulated fluid-absorbing particles.

LCMs can vary in size from 200 mesh to ¾ inch and are typically used atconcentrations of from 8-120 lb/bbl according to the severity of losses.

Cement plugs, often including LCMs, set at the level of lost circulationhave also been used to address these problems during drilling. Lowdensity cements, including foamed cements have been considered asparticularly useful. It has generally been considered that the use ofLCMSs in cement slurries in only effective for minor or partial losses,and the for total loss situations, foamed cement is the only effectivesolution. The most common LCMs used in cement slurries have beengranular materials such as Gilsonite, crushed coal, or ground nutshells. Cellophane flake material has been attempted but problems areencountered with mixing of the slurry at higher loadings. Fibrousmaterials are seldom used in cement slurries because of problems inplugging the cementing equipment. One system using fibers that has beenproposed is described in EP 1284248 and comprises the use of glass orpolymer fibers in a low density cement slurry having solid materialspresent in discrete particle size bands.

Other gelling or viscous systems that do not contain Portland cementhave also been used as plugs. Examples of these are gelling agents suchas silicates with a suitable activator. Such plugs may also containbridging materials such as ground calcium carbonate with particle sizesin the range 8 μm to 254 μm and at concentrations of up to 10 lb (4.5kg)/bbl. Other gelling systems include Sorel cement (magnesium oxide,magnesium chloride and water).

Certain downhole-mixed system have also been proposed. These includemud-diesel-oil-bentonite (M-DOB) plugs and polysaccharide gellingsystems encapsulated in emulsions that are broken by shear forcesdownhole (see EP 0738310). A development of this later system has thegelling system combined with cement (see WO 00/75481)

Further details of lost circulation problems and possible solutions canbe found in Baret, Daccord and Yearwood, Well Cementing, Chapter 6“Cement/Formation Interactions”, 6-1 to 6-17.

It is an object of the present invention to provide techniques that canbe used during drilling to reduce problems associated with lostcirculation.

SUMMARY OF THE INVENTION

In its broadest sense, the present invention comprises treating a wellwith an aqueous base-fluid including solid particles having anequivalent diameter of less than 300 μm and water-dispersible fibershaving a length between about 10 and about 25 mm, added at aconcentration between about 0.5 and 6 pounds per barrel of fluid.

Without wishing to be bound by a theory, it appears that thewater-dispersible fibers enhance the formation of a filter cake byforming a mesh along the borewall that easily plugs with the small solidparticles.

The fluid of the invention can be the drilling fluid itself, and thislater case, the small solid particles are constituted for instance bythe weighting materials added to increase the weight of the drillingfluid, that include for instance barite (barium sulfate), hematite (ironoxide), ilmenite (iron titanium oxide), siderite (iron carbonate),galena (lead sulfide), manganese tetraoxide or zinc oxide. The weightingparticles have an average particle size in the range of 20 to 200μm—that is an order of magnitude of about 100 to 1000 times smaller thanthe water-dispersible fibers. Typically amounts of weighting materialused are in the range 0.2 to 2, more usually 0.25 to 1.5 kg per liter.

According to one embodiment of the invention, the water-dispersiblefibers are glass fibers typically 10-15 mm long, 20 microns in diameter.They are preferably made of alkali-resistant fibers chopped strandshaving a water-dispersible sizing system as products readily availableas asbestos-replacement fibers. The higher the fiber length the betterits ability to form a web-like structure. However, glass-fibers above 15mm have so for proven to be impossible to pump with standard mixingequipment available on a rig site. Concentrations range usually fromabout 1 to about 3 pounds per barrel of fluid though higherconcentration may be required for particularly critical case (in thiscase, the fluid is more likely to be pumped as a pill of relativelyshort volume).

The fibers are typically added in the surface mixing equipment used tomix the drilling fluid. Typical concentrations for the fibers are 1-5lb/bbl. Other LCMs such as other fibrous materials, flakes and granularsized particles can also be added at similar concentrations.

According to another embodiment of the present invention, the fibers arepolymer-fibers such as novoloid fibers, available for instance in lengthranging from about 18 to 22 mm and in diameter of about 21 mm, with awater content of 35-45% (see U.S. Pat. No. 5,782,300 for more detaileddescription of suitable fibers).

As mentioned above, the treating fluid of the present invention can bethe fluid currently used to drill the well, or specially-mixed fluidsfor the purpose of curing lost circulation problems, possibly in theform of a pill of limited volume. In this later case, the fluid willstill contain small-sized solids—typically similar to the one commonlyencountered in drilling fluids. In one embodiment said pill may be aspacer.

One particularly preferred form of pill includes fibers and a cementsuch as micro-cement, optionally with the addition of a bridgingmaterial such as calcium carbonate or sized granular particles. Such apill can include 80% sized calcium carbonate and 20% micro-cement aswell as the fibers. Polymers can also be included.

While such pills can be pumped continuously, it may also be desirable tomix and pump a volume that does not return to the surface but only issufficient only to reach the lost circulation zone so as to avoidplugging surface equipment.

Another aspect of the invention involves the use of the fibers in cementslurries. In this aspect a low-density cement slurry having solidcomponents present in discrete particle size bands, and containing thefibers is prepared and pumped into the well along with pressurized gasso as to form a very low density foamed slurry which is placed adjacentthe lost circulation zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of examples and withreference to the accompanying FIG. 1, which shows a schematic view of asystem for supplying foamed, low-density cements with fibers for lostcirculation problems.

DETAILED DESCRIPTION

The present invention is applicable to various types of drilling fluids,both water based and oil-based as shown in the tables below: CommercialNames Water-Based Muds

Bentonite muds M-I Gel

Polymer muds PolyPlus Inhibitive muds

Encapsulating muds MCAT

Glycol mud Glydrill

Reservoir drilling muds Stardrill DiPro Old Systems

Gypsum mud

Lime mud Exotic water-based muds

Silicate muds SigmaDrill

Conductive muds Sildrill

MMH

Aphrons, FazePro

Formate muds Oil-Based Muds

Diesel-based muds Versadrill

Mineral oil-based muds Versaclean

Low-toxicity mineral oil (LTMO) Versavert Synthetic oil-based muds

Linear alpha-olefins Novaplus

Internal olefins Novatec

Ester muds Petrofree Finagreen

Exotic systems: Acetal, ether, etc. Ecogreen

The following two examples illustrate the use of fiber materials tosolve lost circulation problems in accordance with the invention.

EXAMPLE 1

A well drilled to a vertical depth of 2700 m encountered a severe lostcirculation zone. The well was being drilled with GelChem drilling mudhaving the following properties: Viscosity: 77 PV/YP: 19/15 GelStrength:  6/16 Water Loss: 8.2 pH: 10 Cl: 900 Ca: 35 Density: 1100kg/m³ Plastic Viscosity: 55-65 cp

At the start of the procedure according to the invention, 13 bags(˜295.1 kg) of water-dispersible 10-14 mm long, 20-micron diameter glassfibers were added to the basic mud and pumped into the well. This wasfollowed by a further 8 bags (˜181.6 kg) pumped in the mud, after whichreturns were observed at the surface. Finally, a further 11 bags offibers (˜249.7 kg) were added and pumped into the well after which fullcirculation was observed at the surface (i.e. the amount of mudreturning to the surface equaled the amount pumped into the well). Intotal, 727 kg of fibers were pumped in 110 m³ of drilling mud (GelChem).

EXAMPLE 2

A well drilled to a vertical depth of 630 m encountered a severe lostcirculation zone. The well was being drilled using coiled tubing withGelChem drilling mud with a density of 1050 kg/m³ and a plasticviscosity of 55-65 cp. Initially, LC pills of calcium carbonate orsawdust were pumped with no effect. The treatment was performed in fivestages:

-   -   1. Five bags (˜113.5 kg) of water-dispersible 10-14 mm long, 20        micron diameter glass fibers were added to 12 m³ of the mud and        pumped through the tubing.    -   2. Six bags (˜136.2 kg) of fibers were added to 12 m³ of the mud        and pumped through the tubing.    -   3. Seven bags (˜158.9 kg) of fibers were mixed with the mud and        pumped through the annulus (reverse circulation).    -   4. Twelve bags (˜272.4 kg) of fibers were mixed with the mud and        pumped through the annulus (reverse circulation), after which        returns to the surface in the tubing were observed.    -   5. Eight bags (˜181.6 kg) of fibers were mixed with the mud and        pumped through the annulus (reverse circulation) resulting in        full circulation being observed.

In total 863 kg of fibers were pumped in 70 m³ of mud.

FIG. 1 shows a system in which fibers can be used in conjunction with afoamed, low-density cement slurry to address lost circulation problems.Suitable low-density cements are described in WO 01/09056 (U.S. Ser. No.10/049,198, incorporated herein by reference). The manner in which suchslurries can be foamed in described in WO 00/50357 (U.S. Ser. No.09/914,331, incorporated herein by reference). Low-density cementslurries containing fibers are described in WO 03/014040 (incorporatedherein by reference).

In the system shown in FIG. 1, a base cement blend is prepared in a bulktruck 10. For batch mixed applications, this blend is pumped via apumping truck 12 to a batch mixer 14 at which point fibers are alsoadded. The batch-mixed slurry is then pumped from the mixer 14, viaanother pump truck 16. For continuous mixing, the first pumping unit 12and batch mixer 14 are not required and the fibers (and any otheradditives) are added directly at the pumping unit 16. The output of themixing unit 16 is directed to a foam generator 22 via a check valve 20with a foam stabilizer source 18 and nitrogen source 24 connectedthereto. The foamed slurry is pumped from the generator 22 to thewellhead 26 and down the well in the normal manner. A bypass line 28 andchokes to pits 30 are included as usual.

It will be appreciated that there are numerous variations that can bemade while remaining within the scope of the invention. For example, thefibers can be combined with other LCM's and used in various types ofmuds. The LCM's can be in the form of gelling systems such ascement-containing systems (e.g. calcium carbonate and micro-cement, orthe foamed slurry described above). The fibers can be pumped as a singlepill, as a series of pills, or substantially continuously untilcirculation is restored. The fibers can be pumped through drill pipe,drill bits or other downhole equipment, or through coiled tubing, viathe annulus.

1. A method of treating a well comprising pumping a fluid including anaqueous base, solid particles having an equivalent diameter of less than300 μm and water-dispersible fibers having a length between about 10 andabout 25 mm, at a concentration between about 0.5 and 6 pounds perbarrel of fluid, wherein the fluid is pumped as drilling fluid and thewater-dispersible fibers form a web that enhances the formation of afilter cake upon contact with the wellbore, reducing loss of fluid intothe formation.
 2. The method of claim 1, wherein the fibers have adiameter of about 20 μm.
 3. The method of claim 2, wherein the fibersare glass fibers and have a length of about 10 to about 15 mm.
 4. Themethod of claim 3, wherein the fibers are added at a concentrationbetween about 1 and 3 pounds per barrel of fluid.
 5. The method of claim2, wherein the fibers are polymer fibers having a length of about 18 toabout 22 mm and a water content of 35-45%.
 6. The method of claim 4,wherein the fibers are novoloid fibers.
 7. The method of claim 1,wherein said solid particles are selected from the list consisting ofbarite, hematite, ilmenite, calcium carbonate, iron carbonate, galena,manganese tetraoxide, dolomite, zinc oxide, cement and mixtures thereof.8. The method of claim 2, wherein said solid particles have anequivalent diameter of less than 75 μm.
 9. The method of claim 3,wherein at least 50% of the solid particles have a diameter rangingbetween 10 and 30 μm.
 10. The method of claim 1, wherein the fluidfurther comprises an additional lost circulation material.
 11. Themethod of claim 10, wherein said lost circulation material is selectedfrom the group consisting of fibrous materials, flakes and granularsized particles.
 12. (canceled)
 13. The method of claim 1, wherein thefluid is pumped in the form of a pill of limited volume to remedy losscirculation problems.
 14. The method of claim 13, wherein the fluid ofthe pill comprises cement and a bridging agent as solid particles havingan equivalent diameter of less than 300 μm.
 15. The method of claim 14,wherein said cement is micro-cement and said bridging agent is calciumcarbonate particles.
 16. The method of claim 15, wherein the weightratio of micro-cement to calcium carbonate particles is 80:20.
 17. Themethod of claim 13, wherein said pill is a spacer.
 18. The method ofclaim 1 wherein the fluid is foamed.